Titania Aerogels Research Articles

Optimization of Mass and Light Transport in Nanoparticle-Based Titania Aerogels.

Aerogels composed of preformed titania nanocrystals exhibit a large surface area, open porosity, and high crystallinity, making these materials appealing for applications in gas-phase photocatalysis. Recent studies on nanoparticle-based titania aerogels have mainly focused on optimizing their composition to improve photocatalytic performance. Little attention has been paid to modification at the microstructural level to control fundamental properties such as gas permeability and light transmittance, although these features are of fundamental importance, especially for photocatalysts of macroscopic size. In this study, we systematically control the porosity and transparency of titania gels and aerogels by adjusting the particle loading and nonsolvent fraction during the gelation step. Mass transport and light transport were assessed by gas permeability and light attenuation measurements, and the results were related to the microstructure determined by gas sorption analysis and scanning electron microscopy. Mass transport through the aerogel network was found to proceed primarily via Knudsen diffusion leading to relatively low permeabilities in the range of 10-5-10-6 m2/s, despite very high porosities of 96-99%. While permeability was found to depend mainly on particle loading, the optical properties are predominantly affected by the amount of nonsolvent during gelation, allowing independent tuning of mass and light transport.

Designing Highly Photoactive Hybrid Aerogels for In-Flow Photocatalytic Contaminant Removal Using Silica-Coated Bacterial Nanocellulose Supports.

This study explores the use of silica-coated bacterial nanocellulose (BC) scaffolds with bulk macroscopic yet nanometric internal pores/structures as functional supports for high surface area titania aerogel photocatalysts to design flexible, self-standing, porous, and recyclable BC@SiO2-TiO2 hybrid organic-inorganic aerogel membranes for effective in-flow photo-assisted removal of organic pollutants. The hybrid aerogels were prepared by sequential sol-gel deposition of the SiO2 layer over BC, followed by coating of the resulting BC@SiO2 membranes with a porous titania aerogel overlayer of high surface area using epoxide-driven gelation, hydrothermal crystallization, and subsequent supercritical drying. The silica interlayer between the nanocellulose biopolymer scaffold and the titania photocatalyst was found to greatly influence the structure and composition, particularly the TiO2 loading, of the prepared hybrid aerogel membranes, allowing the development of photochemically stable aerogel materials with increased surface area/pore volume and higher photocatalytic activity. The optimized BC@SiO2-TiO2 hybrid aerogel showed up to 12 times faster in-flow photocatalytic removal of methylene blue dye from aqueous solution in comparison with bare BC/TiO2 aerogels and outperformed most of the supported-titania materials reported earlier. Moreover, the developed hybrid aerogels were successfully employed to remove sertraline drug, a model emergent contaminant, from aqueous solution, thus further demonstrating their potential for water purification.

Oxygenated carbon functionalized TiO2 aerogel surface: Facile synthesis, surface, structural and photovoltaic investigations

The present study reports the synthesis of in-situ surface functionalized TiO2 aerogel through microwave-assisted sol-gel method. Glucose was used as a carbon source, and different amounts of oxygenated carbon were inserted on the surface of TiO2 aerogel. The presence of oxygenated carbon on the surface of TiO2 is confirmed by Raman analysis. The microstructural analysis of the materials was performed by Field Emission Scanning Electron Microscope (FESEM) and transmission Electron Microscope (TEM). After doping with COx, the retained porous surfaces were achieved. X-ray Photoelectron Spectroscopy (XPS) indicates an increase in the amount of carbon and oxygen compared to the titanium and oxygen ratio of TiO2. The increase in the peak area of O1s spectra indicates the insertion of different forms of oxygenated carbon. The C1s spectra confirm that various forms of oxygenated carbon, such as –COH, –COOH, CO–C and OC=O, are on the surface of TiO2 aerogel. Due to surface-adsorbed COx, the TACOx samples had extended absorption band edges and reduced band gaps in optical studies. As a result of that, the energy for the electron injection (0.52 eV) was significantly reduced in the TACOx-20% sample. In addition, the BET surface area analysis reveals a high surface area of 144 m2/g for the TACOx-20% sample. Dye adsorption studies also reveal the high dye loading of 0.65 μmol/mg found in the TACOx-20% sample due to more surface hydroxyl groups on the surfaces. The porous network structure, altered optical properties, and the insertion of hydrophilic groups on the surface of COx-doped titania aerogel are beneficial for adsorbing more dye molecules, which are crucial for improving the photovoltaic efficiency of the materials.

Photovoltaic studies on iodine incorporated titania aerogel nanocomposites

Iodine incorporated titania aerogel nanocomposites were synthesized by the sol–gel method. The nanocomposites were obtained by varying the concentration of iodine as 0, 5, 10, and 20% with respect to the weight percentage of titania. All the synthesized nanocomposites retained the crystallinity and anatase phase, confirmed by X-ray diffraction studies. High-resolution scanning electron microscope images of nanocomposites showed the porous structure with agglomeration by increasing the concentration of iodine. The energy dispersive spectrum revealed the entry of the iodine into the titania aerogel. The absorption edge of the nanocomposites from the UV–Visible spectroscopy showed the red-shifted and thereby reducing the band gap with increasing iodine concentration. The photovoltaic performance of the synthesized iodine-incorporated titania aerogel nanocomposites was performed using as photoanode in the quasi-solid dye-sensitized solar cell and found that the efficiency is increasing with increasing iodine in titania aerogel. The photostability and reproducibility of the device were greater than the pristine aerogel.

Investigation of organic solar cells with highly porous 3D-titania aerogel sensitization, optical and electrochemical properties of metal-free porphyrin dyes with 3-methyl-4-hydroxy phenyl functionality

An anchoring group is necessary for linking dye and photoanode as an array. The inter-electron transfer between the layers produces the photoelectron when it is exciting. The introduction of a novel 3-methyl-4-hydroxy phenyl anchoring group to the porphyrin periphery and its potential use in organic solar cell (OSC) applications were addressed. The bare dyes are soaked with a titania aerogel photoanode. FT-IR, UV–visible, and CV spectral analyses further study the electron transfer mechanism at the TA/dye interfaces. The 3-methyl-4-hydroxy phenyl anchor's two attachment points to the titania aerogel (TA) surface were validated by FT-IR at 1630 and 1400 cm−1. The spectrum broadening and band shift after TA adsorption show the dyes' electronic coupling and aggregation features. In order to provide more insights into the kinetics of electron transfer, the exact HOMO and LUMO energy levels are estimated, and the energy level diagram compares the conductance of TA and electrolyte potential. The 3-methyl-4-hydroxy phenyl anchor was used to demonstrate the ability of the dye/TA photoanode to generate an electrical current as an OSC device. The effects of the subsequent change in photocurrent on the number of anchoring groups in the metal-free porphyrin system are studied.

Effect of deoxycholic acid co-sensitization in porphyrin dye on quasi-solid dye-sensitized solar cells comprising titania aerogel with a large surface area

Herein we have reported the effects of deoxycholic acid (DCA) and porphyrin sensitization on the photovoltaic characteristics of titania aerogel-incorporated quasi-solid dye-sensitized solar cell (QS-DSSC). The obtained results showed that the co-adsorption with DCA results in improving the photovoltaic parameters such as photocurrent density (Jsc), Open-circuit voltage (Voc), fill factor (FF) and overall power conversion efficiency (η). For example, the device treated with DCA (labelled as DCA-1) delivered PCE (η) of 2.56 % (with Voc of 0.61 V), which is greater than those obtained for the unchanged device (labelled as 1) (η = 0.97 %, Voc = 0.53 V). The prime factors responsible for the above enhancement are increased electron injection, suppressed recombination and low electron transport resistance, as confirmed by the analysis of photoluminescence (PL) spectra, dye adsorption measurements, electrochemical impedance spectra (EIS) and time-dependent transient spectra for photocurrent /photovoltage.

Microstructure of titania aerogels by reverse Monte Carlo simulations

The local atomic structure of TiO2 aerogels calcinated at 450 °C and 550 °C has been studied in comparison with that of well-crystallized TiO2 anatase by the X-ray diffraction (XRD) method and Reverse Monte Carlo (RMC) simulations. The influence of calcination temperature on the crystallographic properties and microstructure of TiO2 aerogels was discussed. The large X-ray line broadening of TiO2 aerogels was due to the small crystallite size and lattice strain. From the traditional Williamson-Hall (W–H) analysis results, it was observed that the strain value decreased but the particle size increased as calcination temperature was increased. The TiO2 anatase sample revealed an average particle size of about 62 nm and a very low strain in the nano-crystallite. The experimental X-ray structure factor was derived from the wide-angle X-ray scattering (WAXS) pattern after suitable data reduction. The partial atomic pair correlation functions gij(r), like the gTiO(r), gOO(r) and gTiTi(r) have been revealed from RMC simulation with a fairly good stability and acceptable statistics. The structure model that best describes both the WAXS data consists of nano-crystalline TiO2 with a highly distorted shell and a strained anatase-like crystalline core. The pre-existence of the anatase like core may be critical to the formation of single-phase nano-crystalline TiO2 anatase in crystallization of highly porous TiO2 aerogels upon heating.

Studies on the silver incorporated titania aerogel nanostructure as a photoanode in quasi solid dye- sensitized solar cells

Dye-sensitized solar cells (DSSC) have a significant impact on photovoltaic technology because of their quick, low-cost manufacture, light weight, and high power conversion efficiency (PCE). Numerous research works were undergone on different titania nanostructure used as photoanode in DSSC. High surface area titania nanoarchitecture has effective for more dye absorption to improve the cell performance. Titania aerogels have a high specific surface area and high porosity compared to other titania nanostructures. Incorporation of noble metals into titania aerogel enhanced PCE. In noble metals, silver has promising material because of its cost-effective, less ohmic resistance etc. compared to Au. In this work, we prepared silver doped titania aerogel and studied its photovoltaic performance in the quasi solid DSSC performance. Silver-incorporated titania aerogel was synthesized by the facile sol–gel route. Structural, optical, photovoltaic properties and cell stability of a prepared photoanode material were measured. PXRD revealed that the anatase phase is retained after the incorporation of silver. HRSEM images and EDS spectra confirmed the interconnecting network of titania aerogel and the presence of silver respectively. Due to the Localized Surface Plasmonic Resonance (LSPR) effect, the absorption spectra of the composite exhibited a red-shift around 650 nm. PL spectra showed the decreasing intensity in composites which is the evidence of suppressing electron-hole recombination and improving electron injection. The incorporation of silver has improved the photon absorption capability of the photoanode tends to enhance photocurrent density as well as PCE of QS-DSSC.

Multiphase TiO2 aerogels incorporated with Pd for mixed catalysis in wide UV–Vis spectrum

This paper reports the one-pot synthesis and characterization of multiphase TiO2 aerogels incorporated with Pd nanoparticles as photocatalysts for p-nitrophenol reduction. The junction between palladium nanoparticles and titania aerogels resulted in a catalytic duality, due to the photocatalytic properties of titania through in situ hydrogen formation when excited with UV radiation and the catalytic ability of palladium in forming radical hydrogen on its surface, which dispensed with the use of reagents such as NaBH4 in p-nitrophenol reduction reactions. Additionally, the formation of a trap for photogenerated electrons, a high surface area, the presence of the three crystalline phases of titania (anatase, brookite and rutile) and the antenna effect, together with enhanced the electron mobility allowing its transport to the active sites. This resulted in a delay in electron/hole pair recombination and, consequently, promoted high catalytic efficiency in the UV–Vis spectrum.

Modeling and Simulation of a Dye Sensitized Solar Cell with Porous Aerogel Photoanode

Nanostructured aerogels are potential candidates for photoanodes in dye sensitized solar cells (DSSCs). Recent studies show that solid state DSSC with titania aerogel photoanodes exhibit conversion efficiency of 5.2%. This is mainly attributed to the high short-circuit current density achieved by more dye adsorption and reduced recombination of charge carriers. In this paper we have attempted to develop a simulation based on a theoretical model to understand the performance of aerogel based DSSC. The theoretical results obtained are compared with the experimental results. The results generated by the theoretical model agree well with the results obtained experimentally.

Experimental and Simulation Studies of Platinum-Free Counter Electrode Material for Titania Aerogel-Based Quasi-Solid Dye-Sensitized Solar Cell

Counter electrode is one of the key components in the fabrication of dye-sensitized solar cell (DSSC). It is mainly used to complete the electrical circuit of a solar cell as well as to reduce tri-iodide to iodide at the interface of fluorine-doped tin oxide/electrolyte. Platinum (Pt) is the conventional counter electrode for DSSC assembly. In recent times, lots of research work has been carried out to replace (Pt) with low cost carbon electrodes. In this article, as an alternative to platinum, graphite which is a cost-effective carbon material is used as counter electrode material for DSSC. A DSSC was fabricated using titania aerogel photoanode, graphite counter electrode, gel polymer electrolyte, porphyrin, and N719 dye sensitizers. The photocurrent density-voltage ( J – V ) plots of the fabricated DSSC were measured at one sun illumination. The photocurrent conversion efficiency of graphite-based DSSC was comparable with that of platinum-based DSSC for the N719 dye. The electron transfer process in graphite-based DSSC was explained based on the photo-physical parameters such as relaxation time (τ n ) and diffusion time (τ d ) measurements. The J – V curves were simulated using a MATLAB program to compare with the experimental measurements. There is a good agreement between the simulated and experimental results.

Porphyrin-sensitized quasi-solid solar cells with MOF composited titania aerogel photoanodes

ZnTPP-COOH porphyrin-based quasi-solid dye-sensitized solar cells were fabricated with the following three different photoanode materials: Ni-MOF@titania aerogel composite, Cu-MOF@titania aerogel composite, and pure titania aerogel. Their photovoltaic performances were investigated through photocurrent–photovoltage (J–V) curve, electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent spectroscopy, and transient measurements of photocurrent and photovoltage decay. Among the three cells, the enhancement of photocurrent is observed in cells containing Ni-MOF@titania aerogel photoanodes. As a result, the Ni-MOF@titania aerogel photoanode–based cell achieved the highest photocurrent density of 8.88 mA/cm2 and power conversion efficiency of 2.34%. This value is higher than that of the other two devices. Moreover, the EIS analysis is used to understand better the possible electron transfer behind the photocurrent boosting in Ni-MOF@titania aerogel photoanode–based cell. Low Rct value (44.1 Ω) and low τd value of the Ni-MOF@titania aerogel device are the key factors to improve the electron flow, the Warburg diffusion of electrolyte, as well as the regeneration process. These factors are attributed as the reason for their best performance among other devices. Based on the results obtained, the plausible mechanism for electron transfer is discussed using the energy level cascades of TiO2/MOF/porphyrin dye. The transient measurements indicated that the devices exhibited good reproducibility under multiple illuminations.

Entrapment vs. immobilization of polyoxometalates into silica and titania aerogels: Application in heterogeneous oxidation catalysis

Silica and titania aerogels were successfully used as immobilization and entrapment media for phosphomolybdic and phosphotungstic acids. The hybrid materials were investigated in the heterogeneous oxidation catalysis of phenyl methyl sulfide. The characterization of the hybrid catalysts confirmed the immobilization and entrapment of the polyoxometalates (POM) and their dispersion onto the surface of the solid support. The entrapment of these clusters was found to affect less the surface properties of the inorganic network compared to the immobilization process, thus allowing smoother access to the active sites of the polyoxometalate molecules. The catalytic activity of the POM clusters was widely maintained after immobilization and incorporation, where the chemical nature of the surface functional groups was found to affect the behavior of the catalyst. The fastest oxidation of methyl phenyl sulfide was observed for the reactions catalyzed by POM-incorporated silica aerogels, coupled with a high selectivity towards the methyl phenyl sulfoxide.

Morphology- and Crystalline Composition-Governed Activity of Titania-Based Photocatalysts: Overview and Perspective

Titania photocatalysts have been intensively examined for both mechanism study and possible commercial applications for more than 30 years. Although various reports have already been published on titania, including comprehensive review papers, the morphology-governed activity, especially for novel nanostructures, has not been reviewed recently. Therefore, this paper presents novel, attractive, and prospective titania photocatalysts, including zero-, one-, two-, and three-dimensional titania structures. The 1D, 2D, and 3D titania structures have been mainly designed for possible applications, e.g., (i) continuous use without the necessity of particulate titania separation, (ii) efficient light harvesting (e.g., inverse opals), (iii) enhanced activity (fast charge carriers’ separation, e.g., 1D nanoplates and 2D nanotubes). It should be pointed out that these structures might be also useful for mechanism investigation, e.g., (i) 3D titania aerogels with gold either incorporated inside the 3D network or supported in the porosity, and (ii) titania mesocrystals with gold deposited either on basal or lateral surfaces, for the clarification of plasmonic photocatalysis. Moreover, 0D nanostructures of special composition and morphology, e.g., magnetic(core)–titania(shell), mixed-phase titania (anatase/rutile/brookite), and faceted titania NPs have been presented, due to their exceptional properties, including easy separation in the magnetic field, high activity, and mechanism clarification, respectively. Although anatase has been usually thought as the most active phase of titania, the co-existence of other crystalline phases accelerates the photocatalytic activity significantly, and thus mixed-phase titania (e.g., famous P25) exhibits high photocatalytic activity for both oxidation and reduction reactions. It is believed that this review might be useful for the architecture design of novel nanomaterials for broad and diverse applications, including environmental purification, energy conversion, synthesis and preparation of “intelligent” surfaces with self-cleaning, antifogging, and antiseptic properties.

Bacterial cellulose derived monolithic titania aerogel consisting of 3D reticulate titania nanofibers

Monolithic titania (TiO2) aerogel was fabricated by using bacterial cellulose (BC) as the bio-template and preceramic polymer as titanium resource, via freeze-drying and two-step calcination process. As-prepared TiO2 aerogel BCTi-air-2 h possessed low bulk density (0.04 g/cm3) and moderate mechanical strength. SEM images showed that TiO2 aerogel was composed of 3D reticulate TiO2 nanofibers. The anatase crystalline structure of TiO2 with crystal size around several nanometers was revealed by XRD measurements. N2 adsorption/desorption analysis indicated that TiO2 aerogel possessed high specific area (115 m2/g) and macroporous structure, which contributed to the high photocatalytic activity. Most importantly, the monolithic state will ease the usage, recycling, and regeneration of TiO2 aerogel as photocatalyst.

An approach for highly transparent titania aerogels preparation

Titania aerogels are synthesized using various solvents for sol preparation. The morphology, the optical transmittance, the phase composition and the specific surface area of samples are determined. The sample prepared from isopropanol is found to be white and light-tight and to consist of fibers about 17 nm wide. The fibers are crystalline and have an anatase structure. The specific surface area of the sample is 150 m2/g. The samples prepared from methoxy- or etoxyethanol are transparent yellowish monoliths. They are X-ray amorphous and consist of spherical nanoparticles 1.5–3.5 nm. The specific surface area of these samples is more than 500 m2/g. Transparent titania aerogels are prepared in all-solution approach for the first time.

Oxidation−Stable Plasmonic Copper Nanoparticles in Photocatalytic TiO2 Nanoarchitectures

We describe the incorporation of plasmonic copper (Cu) nanoparticles into titania (TiO2) aerogels and demonstrate surface plasmon resonance (SPR)–driven photoelectrochemical oxidation of methanol under visible light. Copper could provide a more abundant, less expensive alternative to gold and silver as a visible light–active plasmonic metal, however, the propensity of Cu to oxidize at the expense of its plasmonic character greatly limits its potential applications. Most reports of supported, plasmonic Cu nanoparticles describe the necessity of preventing Cu oxidation by maintaining strictly anoxic environments or relying on surface-obscuring chemical stabilizers. We stabilize plasmonic Cu nanoparticles by establishing extensive interfacial contact between Cu nanoparticles and a TiO2 aerogel support. The multiple points of contact between 2–3 nm Cu nanoparticles photodeposited at the networked ~10 nm TiO2 particulates of the aerogel stabilizes Cu against oxidation to an extent that preserves the plasmonic behavior of the nanoparticles, even after long-term exposure to ambient air. The wavelength dependence of photoelectrochemical methanol oxidation at Cu/TiO2 aerogel photoanodes verifies the plasmonic origin of the photoelectrochemistry. Plasmonic behavior is not seen for Cu photodeposited at a commercially available, non-networked TiO2 nanopowder in which the primary particle size is ~10× greater than the size of the supported Cu particle. The divergent behavior of Cu when a 3D Cu||oxide interface is formed on aerogel supports compared to the 1D junction between Cu on commercial TiO2 nanopowder supports highlights the importance of the interfacial design motif on preserving plasmonic Cu.

Chitin Liquid-Crystal-Templated Oxide Semiconductor Aerogels.

Chitin nanocrystals have been used as a liquid crystalline template to fabricate layered oxide semiconductor aerogels. Anisotropic chitin liquid crystals are transformed to sponge-like aerogels by hydrothermally cross-linked gelation and lyophilization-induced solidification. The hydrothermal gelation of chitin aqueous suspensions then proceeds with peroxotitanate to form hydrogel composites that recover to form aerogels after freeze-drying. The homogeneous peroxotitanate/chitin composites are calcined to generate freestanding titania aerogels that exhibit the nanostructural integrity of layered chitin template. Our extended investigations show that coassembling chitin nanocrystals with other metal-based precursors also yielded semiconductor aerogels of perovskite BaTiO3 and CuOx nanocrystals. The potential of these materials is great to investigate these chitin sponges for biomedicine and these semiconductor aerogels for photocatalysis, gas sensing, and other applications. Our results present a new aerogel templating method of highly porous, ultralight materials with chitin liquid crystals.

TiO2 aerogel–Cu-BTC metal-organic framework composites for enhanced photon absorption

Novel composites containing titania aerogel and Cu-BTC metal-organic framework (MOF) were synthesized via sol-gel route and characterized by various techniques. The concentration of MOF in composites was varied between 5 and 20weight% with respect to titania precursor. The presence of prominent peaks of Cu-BTC in the small-angle X-ray diffraction (SAXD) analysis confirmed the incorporation of MOF in the composite. This is further confirmed by Fourier Transform Infrared Spectral analysis (FTIR), energy-dispersive X-ray spectroscopy (EDS) analysis and elemental mapping studies. The Brunauer-Emmett-Teller(BET) surface area was found to vary between 204 and 240m2g−1 depending upon MOF concentration in the composites. It was noted that the pore distribution was found to be narrowed on increasing MOF concentration in the composite. Transmission Electron Microscope (TEM) and Field Emission Scanning Electron Microscope (FE-SEM) images revealed the nanoporous morphology of the composites. The incorporation of MOF to TiO2 aerogel showed a significant increase of absorption in the visible region. These properties suggest that TiO2 aerogel-Cu-BTC composites are promising materials for photovoltaic and photocatalytic applications.

Titania aerogels with tailored nano and microstructure: comparison of lyophilization and supercritical drying

Abstract Structure and phase composition of titania aerogels can be substantially influenced simply by the process of drying their parent water colloid suspensions prepared by the reaction of hydrogen peroxide with suspension of precipitates obtained by neutralization of solution of titanyl sulfate with ammonia. Two methods of drying are compared: (1) lyophilization of fast frozen material immersed in liquid nitrogen, and (2) critical point drying using supercritical CO2 under high pressure. Both methods of drying lead to yellow titanium peroxide aerogels consisting of nanometer-sized blocks. While lyophilization leads to foils consisting of nano-sized crystalline nuclei of peroxo-polytitanic acid dispersed in predominantly amorphous material, the critical point drying provides rather bulk highly porous composite consisting of randomly oriented flat nanoparticles (5–10 nm) composed of crystalline anatase and amorphous peroxo-polytitanic acid.